Lady Diana Spencer was a 19-year-old going on 20 who was infatuated with Prince Charles and in love with the idea of love. She couldn't possibly have known what true love was like, nor what marrying into the royal family would ultimately be like. Reportedly, Diana and Charles were only with each other on twelve occasions prior to marrying. How could they possibly have known anything about each other? Apparently, no one in the royal family was thinking straight at the time. They were all thinking with impossibly old-fashioned minds, and they took a lot of things for granted!
Diana & Charles were completely unsuited in terms of their age difference, their interests in life, their personalities, their goals and their motivations. It's rather mind-boggling that everyone expected a young virginal teenager to simply marry a spineless, needy, lovelorn, sensitive prince, give birth to an heir and a spare and then put up meekly with a loveless marriage, while her husband cheated on her with the woman he should have had the courage and gumption to marry in the first place. I know it would have been difficult back then, but Charles should have laid down the law that if he couldn't marry Camilla, he would not marry anyone.
Perhaps it was fate though for Charles and Diana to endure their miserable marriage, just so their golden offspring could be born.
The two golden offspring weathered their griefs and traumas, and now they have with greater wisdom, honed through the fire of their parents' marriage debacle, met and married remarkable soulmates. There's no need to compare Kate and Meghan with each other, nor against the memory of Diana. All three of these gifted ladies are remarkable in their own individual and unique ways. The young, emotionally needy, unformed girl just out of her teens (and from a broken home) that Diana was when she married in 1981, can not be compared to the savvy, self-assured young women that Kate and Meghan had evolved into by the time of their respective marriages in 2011 and in 2018.
To boot, Duchess Kate and Duchess Meghan both have partners who love them, and they also have strong support systems which include full acceptance by the royal family. Diana actually had none of those valuable resources. It bears remembering that the crucible Diana endured is what has made possible the current solidity and strength of the modern royal family. Of course, the current state of affairs has also been made possible by the Queen's caring wisdom, resilience, adaptability, flexibility and tolerance.